Room Temperature Vulcanization (RTV) Silicone coating
An insulator is an indispensable part of the high voltage transmission system that separates High Voltage conductors from Transmission towers. They are the barriers to leakage current and support conductors in the HV Transmission system. Insulators are mounted outdoors that are exposed to mechanical, environmental, and electrical strains. The electrical strains occur due to the irregular and surge voltages, while the mechanical strains persist due to wind load, oscillation, and weight of the conductors. On the other hand, rainfall, dew formation, UV radiation, and contamination due to environmental pollution were found to be the contributing factors to polymer material degradation and aging. These are considered to be environmental strains on High Voltage insulators.
Environmental contaminants like dust, fumes, corrosive substances, and saltwater accumulate on the surface of the insulators and degrade the insulating integrity of the material. Under wet conditions, this contaminated wet surface provides a path for leakage currents that flow through the surface of the insulator. However, in some areas, Joule heating from leakage current occurs and heats the region to a drier state. This can cause 'Dry Band Arcing' which may lead to a flashover across the insulators. This flashover may temporarily or permanently degrade the insulators. The degradation of polymeric insulators causes a loss of its hydrophobic characteristic, surface flaking, cracks, erosion, punctures on the shed or housing, and worst of all, it allows moisture to penetrate and affect the insulator core.
Recently, the use of Room Temperature Vulcanization (RTV) silicone high voltage insulator coating on the surface of a ceramic insulator has become widespread for reducing the probability of pollution-related flashover of contaminated insulators compared with other methods due to its good dielectric properties, flexibility over a wide range of temperatures, adhesion characteristics. As a result of this coating, routine maintenance and water cleaning of regular insulators reduced significantly.
What is RTV coating?
RTV silicone (room-temperature-vulcanizing silicone) is a type of silicone rubber that cures at room temperature. All known commercially produced variants consist of polydimethylsiloxane (PDMS) polymer, silica nanoparticles (SiO2), alumina trihydrate (ATH), or quartz - to increase the tracking and erosion resistance, a catalyst, and a cross-linking agent. Such materials are dispersed in Naptha , which acts as the carrier to transfer the RTV rubber o the insulator's surface. The coatings are applied on the surface of porcelain and toughened glass insulators. The visual appearance of coatings on the insulator must be smooth, and free of bubbles and lumps, which helps to minimize the contamination collections and prevent the channeling of leakage current. In order to bond well with the surface of the insulators, adhesion promoters are used in RTV coatings. This element adds the most significant property of RTV silicone rubber as insulator coating, hydrophobicity, and migration of the hydrophobicity. This migration can restore the surface hydrophobicity after the layer of contamination builds on it. These water repellent characteristics help to withstand leakage current development. Otherwise, poor adhesion can cause tearing of coating by the wind and may affect the insulator performance.
The electrical and physical properties of the coating systems vary depending on their formulation. These properties are the result of the amount of extending fillers, degree of cross-linkage, and adhesion promotion. Manufacturers recommend the thickness range be kept between 250 µm to 500 µm. This thickness is a practical rule of thumb guide that has been used for years in the coating industry and, as such, has little significance to the coating of electrical insulators. Thick coatings provide increased thermal resistance to the heat generated by dry band arcing and, as such, do not allow the heat to be conducted away from the coated insulator substrate as quickly as thinner coatings. Thick coatings can result in hot-spot temperature rising during dry band arcing, consequently causing thermal degradation of the coating sooner than thinner coatings. On the other hand, a thin coating may degrade rapidly due to wearing from environmental stresses.
With this advanced coating technique, the flashover voltage of contaminated insulators will improve, that will eventually yield a better performance under a polluted environment, reduce economical losses, and offer a low carbon economy.
REFERENCES:
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[12] https://en.wikipedia.org/wiki/RTV_silicone